Strategies towards thermochemical valorisation of spent coffee grounds (SCG): Kinetic analysis of the thermal and thermo-oxidative decomposition

The thermochemical conversion of spent coffee grounds (SCG) under inert and oxidative conditions was comprehensively evaluated as a feasible valorisation route for this biomass. Dynamic thermogravimetric assays were carried out at different heating rates, where the individual contribution of the pseudo-components (hemicellulose, cellulose, lignin) was assessed by a deconvolution approach with Lorentz curves. The kinetic triplet, constituted by the apparent activation energy (Ea), the pre-exponential factor (A) and the model of reaction (g(a)), was defined for each pseudo-component through the application of iso-conversional methods, Master-Curves and Perez-Maqueda criterion. Under an inert atmosphere, the devolatilization of humidity, decomposition of hemicellulose, cellulose and part of lignin, and finally the completion of lignin were identified, resulting in a remaining percentage of 20% of biochar. For hemicellulose, Ea was 217 kJ mol-1, for cellulose 214 kJ mol-1, and lignin 151 kJ mol- 1. Under oxidative conditions, analogous mass-loss stages were found, with the only difference being that biochar was decomposed until only ash (-2%) was obtained as residue. The advantage of using SCG as biofuels, with a high LHV and HHV (-20 MJ kg- 1), was thus demonstrated. The Ea for the hemicellulose was 194 kJ mol- 1, for cellulose 147 kJ mol- 1, and for lignin was 173 kJ mol- 1. Similar decomposition paths were found regardless of the atmosphere for hemicellulose (F4), hemicellulose (D3), and lignin (F2/ 4), which suggests that the temperature, the reactant concentration, the particle shape and the diffusion of products control the decomposition of SCG during non-isothermal pyrolytic or combustive thermochemical processes.

Automated summary

This study comprehensively evaluated the thermochemical conversion of spent coffee grounds under inert and oxidative conditions as a feasible valorisation route for this biomass. The individual contribution of the pseudo-components (hemicellulose, cellulose, lignin) was assessed, and the kinetic parameters were determined. The results demonstrated the advantage of using spent coffee grounds as biofuels, with high heating values. The decomposition paths were found to be similar regardless of the atmosphere.